Definitions
Simple resist polarity
Positive: light will weaken the resist, and create a hole Negative: light will toughen the resist and create an etch resistant mask. To explain this in graphical form you may have a graph on Log exposure energy versus fraction of resist thickness remaining. The positive resist will be completely removed at the final exposure energy and the negative resist will be completely hardened and insoluble by the end of exposure energy. The slope of this graph is the contrast ratio. Intensity (I) is related to energy by E = I*t.Positive photoresist
A ''positive photoresist'' is a type of photoresist in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer. The unexposed portion of the photoresist remains insoluble to the photoresist developer. Some examples of positive photoresists are PMMA (polymethylmethacrylate) single component * Resist for deep-UV e-beam, x-ray * Resin itself is DUV sensitive (slow) * Chain scission mechanism Two components DQN resists: * Common resists for mercury lamps * Diazoquinone ester (DQ) 20-50% weight ** photosensitive ** hydrophobic, not water soluble * Phenolic Novolak Resin (N) ** Frequently used for near-UV exposures ** Water soluble ** UV exposure destroys the inhibitory effect of DQ * Issues: Adhesion, Etch ResistanceNegative photoresist
A ''negative photoresist'' is a type of photoresist in which the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer. The unexposed portion of the photoresist is dissolved by the photoresist developer. * Based on cyclized polyisoprene (rubber) ** variety of sensitizers (only a few % by weight) ** free radical initiated photo cross-linking of polymers * Issues: ** potential oxygen inhibition ** swelling during development *** long narrow lines can become wavy *** swelling is an issue for high resolution patterning * Example: SU-8 (epoxy-based polymer), good adhesion) Modulation transfer function MTF (modulation transfer function is the ratio of image intensity modulation and object intensity modulation and it is a parameter that indicates the capability of an optical systemDifferences between positive and negative resist
The following table is based on generalizations which are generally accepted in the microelectromechanical systems (MEMS) fabrication industry.Types
Based on the chemical structure of photoresists, they can be classified into three types: photopolymeric, photodecomposing, photocrosslinking photoresist. Photopolymeric photoresist is a type of photoresist, usuallyLight sources
Absorption at UV and shorter wavelengths
In lithography, decreasing the wavelength of light source is the most efficient way to achieve higher resolution. Photoresists are most commonly used at wavelengths in the ultraviolet spectrum or shorter (<400 nm). For example, diazonaphthoquinone (DNQ) absorbs strongly from approximately 300 nm to 450 nm. The absorption bands can be assigned to n-π* (S0–S1) and π-π* (S1–S2) transitions in the DNQ molecule. In the deep ultraviolet (DUV) spectrum, the π-π* electronic transition in benzene or carbon double-bond chromophores appears at around 200 nm. Due to the appearance of more possible absorption transitions involving larger energy differences, the absorption tends to increase with shorter wavelength, or largerElectron-beam exposure
Photoresists can also be exposed by electron beams, producing the same results as exposure by light. The main difference is that while photons are absorbed, depositing all their energy at once, electrons deposit their energy gradually, and scatter within the photoresist during this process. As with high-energy wavelengths, many transitions are excited by electron beams, and heating and outgassing are still a concern. The dissociation energy for a C-C bond is 3.6 eV. Secondary electrons generated by primary ionizing radiation have energies sufficient to dissociate this bond, causing scission. In addition, the low-energy electrons have a longer photoresist interaction time due to their lower speed; essentially the electron has to be at rest with respect to the molecule in order to react most strongly via dissociative electron attachment, where the electron comes to rest at the molecule, depositing all its kinetic energy. The resulting scission breaks the original polymer into segments of lower molecular weight, which are more readily dissolved in a solvent, or else releases other chemical species (acids) which catalyze further scission reactions (see the discussion on chemically amplified resists below). It is not common to select photoresists for electron-beam exposure. Electron beam lithography usually relies on resists dedicated specifically to electron-beam exposure.Parameters
Physical, chemical and optical properties of photoresists influence their selection for different processes. * Resolution is the ability to differ the neighboring features on the substrate. Critical dimension (CD) is a main measure of resolution. The smaller the critical dimension is, the higher resolution would be. * Contrast is the difference from exposed portion to unexposed portion. The higher the contrast is, the more obvious the difference between exposed and unexposed portions would be. * Sensitivity is the minimum energy that is required to generate a well-defined feature in the photoresist on the substrate, measured in mJ/cm2. The sensitivity of a photoresist is important when using deep ultraviolet (DUV) or extreme-ultraviolet (EUV). * Viscosity is a measure of the internal friction of a fluid, affecting how easily it will flow. When it is needed to produce a thicker layer, a photoresist with higher viscosity will be preferred. * Adherence is the adhesive strength between photoresist and substrate. If the resist comes off the substrate, some features will be missing or damaged. * Anti-etching is the ability of a photoresist to resist the high temperature, different pH environment or the ion bombardment in the process of post-modification. * Surface tension is the tension that induced by a liquid tended to minimize its surface area, which is caused by the attraction of the particles in the surface layer. In order to better wet the surface of substrate, photoresists are required to possess relatively low surface tension.Positive photoresist
DNQ- Novolac photoresist
One very common positive photoresist used with the I, G and H-lines from a mercury-vapor lamp is based on a mixture of diazonaphthoquinone (DNQ) and novolac resin (a phenol formaldehyde resin). DNQ inhibits the dissolution of the novolac resin, but upon exposure to light, the dissolution rate increases even beyond that of pure novolac. The mechanism by which unexposed DNQ inhibits novolac dissolution is not well understood, but is believed to be related to hydrogen bonding (or more exactly diazocoupling in the unexposed region). DNQ-novolac resists are developed by dissolution in a basic solution (usually 0.26NNegative photoresist
Epoxy-based polymer
One very common negative photoresist is based on epoxy-based polymer. The common product name is SU-8 photoresist, and it was originally invented by IBM, but is now sold bOff-stoichiometry thiol-enes(OSTE) polymer
In 2016, OSTE Polymers were shown to possess a unique photolithography mechanism, based on diffusion-induced monomer depletion, which enables high photostructuring accuracy. The OSTE polymer material was originally invented at the KTH Royal Institute of Technology, but is now sold bHydrogen silsesquioxane (HSQ)
HSQ is a common negative resist forApplications
Microcontact printing
Microcontact printing was described by Whitesides Group in 1993. Generally, in this techniques, an elastomeric stamp is used to generate two-dimensional patterns, through printing the “ink” molecules onto the surface of a solid substrate. Step 1 for microcontact printing. A scheme for the creation of a polydimethylsiloxane (Printed circuit boards
The manufacture ofPatterning and etching of substrates
This includes specialty photonics materials, MicroElectro-Mechanical Systems (Microelectronics
This application, mainly applied to silicon wafers/siliconSee also
* Photopolymer * HardmaskReferences
{{reflist, 30em Lithography (microfabrication) Polymers Materials science Light-sensitive chemicals